In many electrical appliances an uninterrupted power supply (UPS) system is used for securing the operation of the electrical appliance regardless of the disturbances in the power distribution network. The “uninterrupted power supply system” can also be abridged and referred to as the “standby electric supply”.
FIG. 1 presents a general, simple standby electric supply. The AC-supply line 101 connected to the power distribution network is connected to the rectifier units, the number of which can even be only one, but usually there are several units connected parallel to each other. FIG. 1 presents particularly the rectifier units 102 and 103. They produce a certain direct voltage which is supplied through the feed line 104 to the load i.e. to the device 105, the power supply of which is desired to be secured. The feed line 104 has been connected through the switch 106 also to the accumulator unit 107. The system functions so that during the normal operation of the power distribution network the rectifier units 102 and 103 supply electrical power both to the load 105 and to the accumulator unit 107, in which case the accumulator unit remains in charged state. If a disturbance occurs in the power supply network, the accumulator unit starts to discharge to the load 105, the electrical power supply of which is thus not disturbed. When the power distribution network returns to its normal operation, the rectifier units 102 and 103 start again to supply electrical energy both to the load and to the accumulator unit. Then the accumulator unit is recharged with the same amount of electrical charge, which was discharged from it during the disturbance. The switch 106 is opened only under special conditions e.g. when the disturbance lasts long, the accumulator unit has almost entirely discharged and the supplying power cannot be continued without the risk of damaging the accumulator unit.
When the accumulator unit is continuously connected to the charging voltage, this is called permanent charging. The voltage level of the permanent charging has to be selected accurately according to the recommendations of the accumulator manufacturer to ensure as long life as possible for the accumulator unit. It has been found out, however, that closed so called VRLA-accumulators (Valve Regulated Lead-Acid), which are used nowadays generally in accumulator units, tolerate poorly permanent charging compared with the traditional open i.e. flooded lead accumulators. This is thought to be caused by chemical phenomena inside the accumulators, caused by continuous overcharging.
FIG. 2 presents a more advanced so called standby construction, which is otherwise similar to the construction shown in FIG. 1, but a connection and charging device i.e. so called IBCM-module (Intelligent Battery Connection/Charge Module) 201 substitutes for the separating switch of the accumulator. It has a control connection to the accumulator unit 107 from it (presented by a narrow line in the figure) and it can take the charging energy of the accumulators either directly by bypassing the rectifiers from a connection by which the standby system is connected to the power distribution network (presented by a dashed line in the figure) or from the voltage supplied to load generated by the rectifiers. During the normal operation the IBCM-module 201 keeps the accumulator almost continuously separated from the direct voltage supplied by the rectifier units 102 and 103 i.e. from the feed line 104. If the voltage in the feed line 104 falls below the threshold value e.g. because of the disturbance occurring in the power distribution network, the IBCM-module 201 connects the accumulator unit 107 to the feed line 104, in which case the load 105 continues to get electric power.
The charge of the accumulator is discharged slowly by itself also when the accumulator is not connected anywhere. If the IBCM-module 201 detects that the charge of the accumulator 107 has under normal operation dropped below a certain threshold value, it connects the accumulator unit 107 either to the feed line or through a separate rectifier (not shown in FIG. 2) to the power supply network, in which case the accumulator unit is fully charged relatively quickly. After this the IBCM-module 201 disconnects the accumulator unit again from the charging voltage. It has been assumed that using the IBCM-module can even double the life of the accumulator unit.
The problem in the system according to FIG. 2 is finding the right control algorithm for the IBCM-module. A wrong algorithm can even lead to poorer operation of the system and that it wears out the accumulators more than the simple system according to FIG. 1. In addition, a wrong algorithm can contribute in increasing production costs e.g. if the components of the device must therefore be dimensioned for unnecessarily high current.